1. Field of the Invention
The present invention relates to a three dimensional shape measurement apparatus, three dimensional shape measurement method, and computer program.
2. Description of the Related Art
Various three dimensional (3D) shape measurement methods have been proposed. Upon roughly classifying these methods, they include a passive type, which performs a shape measurement using only a capturing device without using any illumination device, and an active type, which uses an illumination device and capturing device in combination. The active type assures higher robustness against an object than the passive type, and can attain a high-precision distance measurement. Even when surface texture information of an object is small, the active type is hardly influenced by the surface texture information since it attains a shape measurement based on a projected illumination pattern. Typical examples of the active type include a method based on triangulation, a method based on phase information variations, and a method based on defocus information of a projected pattern.
Of these methods, the method using phase information variations of a pattern is called a fringe pattern projection method, and as one of typical methods, a Fourier transformation method is available (see M. Takeda and K. Mutoh, “Fourier transform profilometry for automatic measurement of 3-D object shapes,” Appl. Opt. Vol. 22, No. 24, p. 3977-3982 (1983)). However, with this method, since phase variations that can be measured are limited to those within a specific area, when a shape change outside the area takes place, phases are aliased, thus requiring phase unwrapping. Hence, it is relatively easy for an object having a smooth shape to attain phase unwrapping using phase continuity. However, it becomes difficult to attain phase unwrapping in case of an object having an abrupt shape change or discontinuous objects.
As the method based on defocus information of a projected pattern, a shape measurement method using a defocus caused by a capturing optical system (see Japanese Patent No. 3481631) and a shape measurement method using a defocus caused by a projection optical system (see Li Zhang and Shree Nayar, “Projection defocus analysis for scene capture and image display,” ACM Trans. on Graphics, Vol. 25, 3, P. 907-915 (July 2006)) are available. In the latter method, when a capturing optical system is a pan-focus system, a blur becomes larger in proportion to defocus amounts from a focal point position of a projection optical system. This blur is evaluated as a spread of a pattern on an image, and its shape is measured. More specifically, 24 different patterns obtained by finely shifting a rectangular projection pattern are projected and their images are captured. Defocus amounts are calculated based on luminance value variations by time-serially arranging luminance values of respective pixels of images for respective patterns. This method is not suitable for a real-time measurement since many patterns have to be projected, although depth absolute values can be calculated.